Process of treating rubber



Patented Feb. 7, 1939 V UNITED STATES PATENT OFFICE PROCESS OF TREATING RUBBER Emil W. Schwartz and Evan T. Croasdale, Bridgeport, Conn., assignors to General Electric Company, a corporation of New York No Drawing. Original application September 16,

1936, Serial No. 101,075. Divided and this application June 12, 1937, Serial No. 147,990

5 Claims.

adapted for use in making rubber compounds for insulating electrical conductors and cables to be operated in wet locations.

It has been known heretofore that rubber can be deproteinized, that is, substantially freed of proteins, and that the use of deproteinized rubber in rubber compounding improves the waterresisting properties of the vulcanized or unvulcanized end-product. We make no broad claim to the use of deproteinized rubber as an ingredient of any rubber compound, but we do claim as new and novel the improved procedure we have developed for treating rubber to deproteinize and otherwise improve its useful properties and characteristics. Such improved deproteinized rubber advantageously may be employed in producing rubber insulating compound, such as hereinafter more fully described, and as particularly claimed in the parent application.

The novel features which are characteristic of our invention are set forth in the appended claims. The invention itself, however, will best be understood from the following specification:

Rubber insulating compound produced in accordance with the invention claimed in the parent application comprises deproteinized rubber in an amount such, for instance, as about 29 per cent by weight and the remainder consisting of finely divided zinc oxide, finely divided inert filler, antioxidant, plasticizer and an organic accelerator capable of yielding nascent sulfur at vulcanization temperatures, for example, a thiuram polysulfide. More particularly, it comprises materials ofthe kinds just described advantageously combined in the following proportions:

(1) Parts by weight Deproteinized rubber 29. 0-34. 0 Finely divided zinc oxide (dried) 23. 0-28. 0

Finely divided inert filler (dried) 37. 0-44. 0 Anti-oxidant 0. 3- 2. 0 Thiuram polysulfide 0. 5- 2. 0

Or, still more specifically:

A small amount of plasticizer, for example,

about 0.5 to 2.0 parts by weight, may be incorporated with the other ingredients, in the stated range of proportions, in the formulas given above.

If desired a small amount, for example, about 0.5 to 3.0 parts by weight, of carbon black, for instance such carbon blacks as those known to the trade as P 33. Gastex and Micronex (and preferably the softer blacks such as P 33 or Gastex) may be a part of the finely divided inert filler of the formula given under (1), the remainder of the inert filler advantageously being whiting and clay of the kinds used in rubber compounding.

Fillers or filling materials are those substances in powder form that are incorporated in rubber compounds for the purpose of increasing the bulk of the compound. They are'usually inert substances which generally produce strengthening effects on the vulcanized compound. In the preparation of rubber insulation for wires and cables, the kind and amount of filler have a material infiuence upon the properties of the end-product. Examples of inert fillers that may be used in practicing this invention are whiting, blanc fixe, clay and asbestine.

' In rubber compounding certain finely divided substances or fillers which, when properly dispersed in rubber, provide the vulcanized product with improved physical properties, for example, greater energy of resilience, greater resistance to abrasion, higher modulus of elasticity and tensile strength, are often described as reinforcing agents. Examples of such substances are zinc oxide, channel carbon black and socalled soft blacks. Magnesium carbonate in an amount up to about 5 per cent by weight of the whole may also be used. Higher amounts of magnesium carbonate detrimentally affect theuseful physical properties of a rubber compound, for example, by stiffening the compound excessively. Substances of the kind just stated comprise a part of the total filler content of the rubber compound produced by this invention. We use zinc oxide as one of such substances both to activate the particular thiuram polysulfide employed as an accelerator of vulcanization and to add a certain degree of strength and heat resistance to the end-product.

An anti-oxidant is a substance that inhibits or retards oxidation catalytically or by preferential absorption of oxygen. Anti-oxidants used in rubber compounding are usually synthetic organic substances. Examples of anti-oxidants that may be used in preparing our improved rubber insulating composition are those known to the trade as Neozone D, Neozone E, Antox and B. L. E.. Neozone D is phenyl-beta-naphthylamine; Neozone E" consists of about 75 per cent phenyl-beta-naphthylamine and about 25..

per cent metatoluylenediamlne oxalate; Antox and B. L. E. are aldehyde-amine reaction products.

An accelerator is any substance that hastens the vulcanization of rubber, causing it to take place in shorter time or at lower temperature or both. According to potency, or speed of action, accelerators are commonly classed by those skilled in the art as slow, medium, semi-ultra and ultra accelerators. Thiuram polysulfides are commonly referred to in the trade as ultra accelerators, but actually they are rapid accelerators of vulcanization only in the presence of added sulfur. Thiuram polysulfides, for example, tetra-methyl-thiuram disulfide, known to the trade as Tuads, and di-pentamethylenethiuram tetrasulfide, known to the trade as Tetrone A, split off nascent sulfur at vulcanization temperatures. The rubber compound of this invention therefore is cured without the addition of any other free sulfur.

Substances which soften a rubber compound and make it easier to mold or extrude are commonly known as plasticizers. Examples of plasticizers which may be used in practicing this invention, and which are mentioned for purpose of illustration only, are stearic acid, zinc laurate, vegetable oils such as palm oil, China-wood oil, linseed oil and the like, mineral oils and waxes, et cetera. Such substances also generally function as dispersing agents.

For plasticizing the rubber, as a means for promoting the rapid and uniform dispersion of the solid ingredients throughout the mass, and as a lubricant of the dies during the application of the rubber compound to a conducting core by extrusion means, a combination of parafin wax and stearic acid is especially effective. Thus, with the other ingredients in a formula such as the one given immediately hereinafter, a plasticizer consisting, for example, of 1.0 part of paraifin wax and 0.25 part of stearic acid may be used. Green ozokerite, after being strained to free it of impurities, is also a suitable plasticizer. Thus we may use as a plasticizer a mixture of ozokerite and stearic acid, or a mixture of ozokerite, paraffin wax and stearic acid.

In order that the invention claimed in the parent application may be more fully understood and practiced by those skilled in the art to which it pertains the following specific example thereof is given, it being understood that it is merely illustrative in nature.

Parts by weight Deproteinized rubber 30. 0 Finely divided zinc oxide (dried) 26. 0 Finely divided clay (dried) 14.5 Finely divided whiting (dried) 27. 0 Carbon black 0. 7 Anti-oxidant 1 0.6 Tetra-methyl-thiuram disulfide 1.2

About 70 per cent of the total deproteinized rubber of a particular batch is first ground in a suitable machine, for example, a Banbury mixer, for about 3 minutes, after which the fillers and anti-oxidant are incorporated therewith, and the mixing continued for about 5 minutes. The remainder of thedeproteinized rubber is now added and the whole mixed for a further period, say, about 8 minutes. The mass is then mixed on a rubber mill for about 5 minutes, after which it is strained and then aged for a few days. When ready to use, the tetra-methyl-thiuram disulfide is'added to the stock prepared in the manner described, and the whole thoroughly mixed on a mill.

The stock is extruded in well-known manner on conductors by means of standard tubing practips; or it may be sheeted on a calendar and applied in the form of a tape to any electrical conductor or cable which may be conveniently insulated by such means. The rubber insulation is then vulcanized in place on the conductor. In the case of a No. 14 (0.064 inch diameter) copper conductor covered with a 3/64-inch wall of rubber, the ccmpound'may be vulcanized by employing, for example, a 45-minute rise to about 25 to 35 pounds steam pressure (l30.5 to l38.3 C.) and a 45-minute cure at such steam pressure, plus or minus 15 minutes variation either in the time in reaching the desired curing temperature or in the time of curing at such temperature, or in both the time in reaching the curing temperature, and in the time of curing thereat. As is well-known to those skilled in the art, the time required for effectively curing rubber insulation in placeon electrical conductors depends both upon the wall thickness of the rubber and the diameter of the conductor. The greater the wall thickness and the diameter of the conductor, the longer is the curing time required.

In producing the rubber insulating compound and insulated conductor claimed in the parent application we prefer to use natural rubber that has been treated, to deproteinize and otherwise improve it, in a manner such, for instance, as the following:

A convenient amount for handling, for example, about pounds of No. 1 smoked sheet rubber is creped very finely on a single pass through a tight rubber mill. The creped rubber is put in a basket and placed in an autoclave, which is then about two-thirds filled with hot water. The lid of the autoclave is clamped on, and steam introduced until the rubber is under an absolute pressure of about pounds per square inch (gauge pressure of about 125 pounds per square inch). This pressure represents a temperature of about l78.3 C. (353 F.). During pressure rise the exhaust valve is opened about every 5 minutes to assure the removal of all air from the autoclave. When a gauge pressure of about 125 pounds per square inch has been reached, the steam is turned off and the autoclave'is directly fire-heated using gas as fuel. The gauge pressure is brought to about pounds per square inch and held there by automatic control for approximately 1 to 2 hours, for example, for about 1% hours. A gauge pressure of 150 pounds per square inch (absolute pressure about. pounds per square inch) represents a temperature of about 185.5 C. (366 F.).

.The maximum temperature of treatment is critical. Heating the rubber to a temperature above about 205 detrimentally affects the useful physical properties of the rubber as a result of reversion or depolymerization of the rubber hydrocarbon. When efforts are made to use temperatures below about C. (347 F.) in the digestion treatment,excessive periods of time of digestion are required. Obviously, the longer the time of autoclaving, the more costly the process.

There, is therefore a certain minimum temperature that is critical in the sense that, if a substantially lower temperature were used, the treatment would be so uneconomical as to make the process of no practical success. For the reasons stated we therefore advantageously maintain the the end of the washing operation, as a dryer.

It is important that the autoclaved rubber be kept totally immersed in water pending, and durihg, its transfer to the 'washer in order to avoid the detrimental effectof air upon the rubber. Hot water is run into the washer continuously, the overflow line being kept open to allow free circulation of water. Steamat low pressure, for instance, at about 15 poundsp-ressure, is turned into the jacket and rolls of the washer so that the water is kept boiling during the washing operation. Live steam, if desired, may be passed through the water, in the washer. Boiling the water, in addition to facilitating the removal of water-soluble digestion products, also drives out any oxygen absorbed by, or entrapped inthe water. V

A novel and important feature of our washing operation consists in bubbling a non oxidizing gas, that is, a .gas in which rubber is insert, through the water during the washing operation. Gases such as hereinafter described with reference to the drying operation may be used for this purpose. Such procedure aids in preventing degradation of the rubber during washing. The inert gas also assistsv in removal of oxygen-containing gases from the hot wash water and in providing a non-oxidizing gaseous atmosphere within the washer. The rubber is continuously washed with the washer lid in closed position, until the wash water shows no color. Ordinarily, washing for about one hour is sufficient.

At the end of the washing period the water is drained from the washer and non-oxidizing gas, that is, a gas in which rubber is inert, is passed into the washer while continuing the introduction of steam at a temperature above 212 F. for the purpose of drying the rubber. Nitrogen, hydrogen, carbon dioxide, producer gas, or mixtures of such gases, areexamples of non-oxidizing gases that may be used.

' "A suitable amount of an-anti-oxidant such, for

example, as Neozone A, Neozone D, Neozone E, Antox, .B. L. E., or a mixture of such antioxidants, is now incorporated with the washed, but undried, rubber. The autoclaving operation removes very powerful and useful anti-oxidants naturally occurring in crude rubber. We have found that the otherwise detrimental effect of this loss of naturally occurring anti-oxidants upon the useful properties of the rubber may be corrected conveniently and most effectively by compounding a synthetic organic anti-oxidant with the washed rubber prior to the drying opera ation. About 18 ounces of an anti-oxidant such as Neozone A (phenyl-alpha-naphthylamine). is sufficient for effectively treating a 1l0-pound batch of rubber. Addition of anti-oxidant at this stage also greatly improves the coherence of the purified rubber. We have found that it also aids in preventing degradation of the rubber during subsequent operations, for instance, the drying operation. Addition of a synthetic anti-oxidant to the digested and Washed rubber prior to the drying operation is accordingly an essential and important feature of our improved process.

Having added the anti-oxidant, the lid of the washer is clamped shut. After about 10 minutes the steam is turned off the rolls. Non-oxidizing gas is passed into the washer during the entire drying operation. When producer gas is the nonoxidizing gas, a pilot light is kept burning at the exhaust opening. The producer gas is checked at regular intervals to be sure that it is essentially free'from oxygen. The deproteinized rubber is dried by this means, at a temperature of about 110 to 125 0., in approximately 20 minutes after the water has been drained from the washer. 1

The dried rubber is removed from the washer and slabbed on cooling. The purified rubber is darkin color. When hot it is very plastic; on cooling it becomes about as elastic assmoked sheet rubber that has been worked on a rubber mill for a short time. The treated rubber is aged in storage for several days prior to use.

The process just described provides a commercially successful means for treating rubber to deproteinize it, and otherwise to improve its useful properties. Tests on the commercially produced deproteinized rubber show as low as 0.13 per cent nitrogen, which, when the protein content is calculated as 6.25 times the nitrogen content, represents about 0.8 per cent protein. Obviously, the extent of the reduction in protein content is dependent upon the protein content of the starting rubber, but in general it may be stated that by the described procedure the nitrogen content of ordinary crude rubbers is reduced to less than 0.3 per cent. The process depends for its practical success upon washing the digested rubber to remove water-soluble matter, such as protein-digestion products, sugars, soluble-inorganic salts, as well as any mechanical impurities, rapidly, efficiently and economically, without degrading the rubber in any way such, for instance, as by exposure to oxygen or to excessive mechanical working; and, further, upon effectively'and quickly drying the washed rubber likewise without degradation.

As a modification of the above-described process, We may digest the rubber in a hot dilute solution of sodium chloride or in a weak alkali solu-. tion, for instance. a solu ion of sodium hydroxide, the subsequent washing and drying operations being essentially the same as when water alone is used. Thus, to facilitate solubilization 'of protein substances of the rubber, we may use in the digestion treatment of 100 pounds of crude rubber a dilute aqueous solution containing, for example, 2 pounds of sodium chloride; or. for the same amount of rubber, a dilute aqueous solution con taining, for instance, 300 grams of sodium hydroxide. In certain cases, too, for instance, when it is desired to obtain a rubber of even lower nitrogen content, an alcohol wash or washes may be applied to the rubber after the water wash.

Such alcohol wash may be used after the water wash of rubber digested in the presence of water alone, or in aqueous solutions of sodium chloride, sodium hydroxide, or other agents having a solubilizing effect upon protein substances contained in crude rubber. By the methods just described, we have reduced the nitrogen content of crude rubber to as low as about 0.05 to 0.10 per cent; and. in the case of crude rubber having aninitial nitrogen content of about 0.30 per cent, to as low as about 0.02 per cent.

The superior and characteristic properties of rubber compound produced in accordance with this invention will immediately be appreciated by those skilled in the art from the following descriptlon of the product, based on the results of tests made by methods approved by the trade:

A 2-inch mark on a 6-inch test piece of a rubber compound made as herein described stretches at least 400 per cent before breaking. The set in a 2-inch mark on a 6-inch test piece one minute after release is not greater than inch. The initial tensile strength of the compound is at least about 1300 pounds per square inch. The procedures for making elongation, set and tensile strength tests are described under specification. D-27-35T of the American Society for Testing Materials. The product conforms in all respects to the electrical requirements of a rubber insulation as set forth in said specification D -2'7-35T.

Further, when a sample of vulcanized rubber compound produced in accordance with this invention has been subjected to a Bierer-Davis oxygen-bomb test for 21 days under a pressure of 300 pounds per square inch and at. 70 C., it shows a depreciation from that of the original compound of not more than 25 per cent in elongation and in tensile strength.

The low moisture-absorbing properties of a compound produced in accordance with this invention may be established in the following manner:--

A 12-inch length of the insulated conductor, with all coverings removed, is bent in the shape of a U. The ends of the sample are inserted in the holes of a stopper that fits tightly a receptacle filled with distilled water. The insulated conductor is so adjusted that a length of 9 inches is submerged in the distilled water when the stopper is inserted in the container. After immersion for 7 days in the distilled water, which is maintained at a temperature of 69 to 71 C., the sample is removed, wiped free of surface water and weighed. It will be found that the increase in the weight (moisture-absorption factor) of the rubber insulation is less than 0.010 gram per square inch of the exposed surface area.

The following more specific data are given as illustrative of the improved moisture-resisting properties of rubber compounds claimed in the parent application and wherein is utilized rubber treated as claimed in the present application.

Water-absorption test, 7 days in distilled water, temp. 69-71 C.

Rubber compound produced in accordance with this invention has particular utility as insulation for cables subjected to excessive moisture conditions, for example, non-leaded submarine cables, non-metallic parkway cables, and the like.

By the term deproteinized rubber as used herein we mean rubber which has been treated to solubilize protein substances contained in crude rubber, which solubilized substances are subsequently removed from the treated rubber.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process of treating crude rubber to improve its useful properties which comprises converting protein substances contained in the starting rubber to a water-soluble state, washing the resulting rubber with aqueous liquid in a nonoxidizing gaseous atmosphere until said rubber is essentially free from water-soluble constituents, passing non-oxidizing gas through the aqueous wash liquid during the washing operation, and drying the Washed rubber.

2. A process of improving the useful properties of crude rubber which comprises treating the starting rubber to convert protein substances present therein to water-soluble state, washing the resulting rubber with hot water until said rubber is essentially free from water-soluble constituents, passing non-oxidizing gas comprising producer gas through the hot wash water during the washing operation, adding a synthetic antioxidant to the washed rubber, and drying the moist rubber in intimate contact with said antioxidant and in a non-oxidizing gaseous atmosphere.

3. A process of treating crude rubber to improve its useful properties which comprises digesting, in a closed vessel, finely creped rubber in the presence of water and in a non-oxidizing gaseous atmosphere at a temperature and pressure sufficiently high and for a period of time sufficiently long to reduce protein substances contained in the crude rubber to a water-soluble form without degrading said rubber, continuously washing the digested rubber with hot water in a non-oxidizing gaseous atmosphere until said rubber is essentially free from water-soluble constituents, passing a non-oxidizing gas through the hot wash water during the washing operation, adding a synthetic anti-oxidant to the washed rubber, and drying the moist rubber in intimate contact with said anti-oxidant and in a nonoxidizing gaseous atmosphere.

4. In a process of treating crude rubber to improve its useful properties which comprises converting protein substances contained in the starting rubber to water-soluble state, washing the resulting rubber with aqueous liquid to remove water-soluble constituents and drying the washed rubber, the improvement which consists in passing a non-oxidizing gas through the aqueous wash liquid during the washing operation and drying the moist, washed rubber in the presence of an added synthetic anti-oxidant and in a non-oxidizing gaseous atmosphere.

5. In a process of improving the useful properties of crude rubber which comprises treating the starting rubber to convert protein substances therein to water-soluble state, washing the resulting rubber with hot water to remove watersoluble constituents and drying the moist, washed rubber in an atmosphere of producer gas, the improvement which consists in passing producer gas through the hot wash water during the washing operation and drying the moist, Washed rubber in the presence of an added synthetic antioxidant.

EMIL W. SCHWARTZ.

EVAN T. CROASDALE. 

